This invention relates to a fuel cell system and more particularly to a combustor which heats a fuel processor which produces an H2-rich feed gas for consumption in a fuel cell stack.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2).The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors such as a water/gas shift (WGS) and preferential oxidizer (PROX) reactors are used to produce carbon dioxide (CO2) from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2. A combustor typically is included in a fuel cell system and is used to heat various parts of the fuel processor, including reactors, as needed.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in U.S. patent application Ser. No. 08/975,422, which corresponds to U.S. Pat. No. 6,232,005 issued on May 15, 2001, in U.S. Ser. No. 08/980,087, which corresponds to U.S. Pat. No. 6,077,620 issued on Jun. 20, 2000, and in U.S. Ser. No. 09/187,125, which corresponds to U.S. Pat. No. 6,238,815 issued on May 29, 2001, each of which is assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control operation of major interdependent components or subsystems such as the combustor and fuel processor. This is particularly difficult during start-up of a vehicular fuel cell system wherein the combustor heats up the fuel processor to a temperature sufficient for the fuel processor to generate hydrogen-rich feed for the fuel cell. It is also difficult to maintain combustor heat during transition from a start-up mode to a running mode where the combustor is at least partially fueled by the hydrogen-rich stream from the fuel processor.
Therefore, it is desirable to provide a method by which the fuel cell system is effectively operated during start-up and a running mode so that the combustor and fuel processor are operated efficiently.
The present invention is directed to the operation of a fuel cell system which comprises a combustor which heats a fuel processor which, in turn, generates a hydrogen-rich feed stream for use in a fuel cell stack. The hydrogen-rich feed stream is consumed in the fuel cell stack whereby electricity is produced. The present invention provides a new method for operating the combustor within the system and, particularly, an improved method for operating the combustor during start-up and transition to a running mode. In another aspect, the invention provides an improved system configuration and apparatus whereby start-up is achieved in a fuel efficient and effective manner.
In one aspect, the invention provides a method for operating a combustor to heat a fuel processor to a desired temperature during start-up in a fuel cell system. Here the term start-up indicates that the fuel processor is started from a relatively cold condition. Start-up includes commencing operation after the fuel processor was cooled down to below its desired operating temperature. The fuel processor generates a hydrogen-rich product (feed stream) from a hydrocarbon. The hydrogen-rich feed stream from the fuel processor is supplied to a fuel cell stack which generates electricity by oxidation of the hydrogen with oxygen. In a preferred start-up mode, a hydrocarbon fuel stream and an air stream are supplied to the combustor. The hydrocarbon fuel and air are reacted or burned in the combustor in order to generate heat to heat the fuel processor. The pressure of the air stream to the combustor is monitored. The products of the combustion reaction in the combustor are supplied to the fuel processor.
Preferably, the fuel processor is heated by indirect heat transfer from the products of combustion. After the products of combustion from the combustor have begun to heat the fuel processor, a hydrocarbon reactant is supplied to the fuel processor. The hydrocarbon reactant is reacted with steam, air, or a combination of both in the fuel processor. The reaction between the hydrocarbon reactant and the steam and/or air produces a hydrogen-rich feed stream which is usable in the fuel cell stack to produce electricity. However, at the outset of the fuel processing in the fuel processor, the hydrogen-rich (H2) feed stream is often not of a quality suitable for the generation of electricity. Therefore, the hydrogen-rich stream may be directed in a flow path from the fuel processor directly to the combustor.
Initially, the flow path from the fuel processor to the combustor is at a relatively low pressure as compared to the pressure of the air stream being supplied to the combustor. Therefore, it is desirable during start-up to permit the pressure in the flow path from the fuel processor to the combustor to increase so that the pressure in the flow path to the combustor becomes greater than the pressure of the air stream in order to prevent back feed of the air stream into the flow path. By the method of the invention, the flow path from the fuel processor into the combustor remains closed until such pressurization has occurred. Thereafter, fluid flow communication from the flow path into the combustor is initiated whereupon the hydrogen-rich feed stream is admitted into the combustor for reaction therein with the air.
After the H2-rich feed stream is admitted into the combustor, it is necessary to decrease the combustor""s supply of the hydrocarbon fuel stream to regulate the generation of heat in the combustor. In a preferred aspect of the invention, the decrease in the supply of the hydrocarbon fuel stream is accomplished by progressively decreasing such supply in such a manner that the fuel input (FI) at time n is proportional to (1xe2x88x92K)xc3x97FI1xe2x88x92n. In one aspect, the K value remains a constant. The K value is selected or predetermined according to the dynamics of the system. In another aspect, the K value is selected from a look-up table where K varies over time. As can be appreciated, the method of the invention provides for flexibility in establishing the phasing-out of the supply of the hydrocarbon fuel stream to regulate the generation of heat in the combustor. In still another aspect of the invention, the step of decreasing the supply of the hydrocarbon fuel stream into the combustor does not occur immediately once the pressure in the flow path becomes greater than the pressure of the air stream. Rather, a time delay is implemented between the time the aforesaid pressure criteria is met and the decreasing of the supply of the hydrocarbon fuel is initiated. This controlled supply of H2 rich stream and decrease of hydrocarbon fuel to the combustor provides a smooth and efficient transition into a running mode while substantially maintaining a desired level of heat output from the combustor.
In a preferred aspect, the above start-up mode of operation is accomplished by placing a check valve in the fuel cell system which is located in the flow path followed by the hydrogen-rich feed stream into the combustor. The pressure in the flow path is monitored upstream of the check valve. A second pressure monitor is located in the flow path of the air stream. Since the flow path of the air stream and the hydrogen-rich feed stream combine together in the combustor, the check valve provides an effective means to block the flow path of the hydrogen-rich feed stream into the combustor until the pressure of the hydrogen-rich feed stream exceeds the pressure of the air stream whereby the check valve is opened and permits the desired flow of hydrogen-rich feed stream and air into the combustor during start-up.
In another aspect, after the fuel processor has attained and maintained its desired temperature, it produces the hydrogen-rich stream which is consumed in the fuel cell stack to produce electricity. However, the quantity of hydrogen supplied to the fuel cell stack is greater than that required to produce the increment of power desired from the system, therefore, at least a portion of the hydrogen-rich feed stream is not consumed in the fuel cell stack and is directed to the combustor. This excess portion of the hydrogen-rich feed stream is reacted with the air stream in the combustor for generation of heat which is thereafter supplied to the fuel processor.
In a preferred aspect therefore, the fuel cell stack is arranged in the flow path between the fuel processor and the combustor and two variations are possible. In one variation during start-up, the hydrogen-rich feed stream produced by the fuel processor, which is initially of low quality, is directed to a flow path from the fuel processor directly to the combustor. In another variation during start-up, before the fuel cell stack begins to produce power, such low quality hydrogen-rich feed stream is supplied from the fuel processor through the fuel cell stack, where it is not consumed, and passes there through and is then directed to the combustor.